This invention relates to active matrix electro-optic display devices, and more particularly relates to such devices having a light shielding layer associated with the matrix, and also relates to color projection apparatus incorporating these devices.
In one form of color projection television (PTV) in widespread use today, a full color display is formed by superimposing three different (red, blue and green) monochrome images on a projection screen. While these monochrome images are usually formed using cathode ray tubes (CRT's), it has also been proposed to use active matrix electro-optic displays such as thin film transistor (TFT) switched twisted nematic liquid crystal displays (TN LCD) in place of the CRT's to generate these images.
In a TN LCD cell (sometimes referred to as a "light valve"), operating in the transmission mode, a liquid crystal (LC) material is sandwiched between two optically transparent walls, the inner surfaces of which each carry an orienting layer in contact with the LC, for promoting a preferential alignment of the LC molecules adjacent to the layers. Because the LC molecules also tend to align with one another, as well as with the orienting layers, a twist can be imparted to the LC by assembling the cell with a non-parallel orientation of these layers. For example, arranging the layers with their alignment directions orthogonal to one another produces a ninety degree twist of a nematic LC from one surface to the other. This twist enables the LC to rotate linearly polarized light by ninety degrees, so that the amount of light transmitted by the device can be controlled by an exit polarizer, called an analyzer. Furthermore, the LC can be untwisted by application of a voltage, so that, for example, light blocked by an analyzer having its polarization direction oriented orthogonally to that of an entrance polarizer, can be passed by application of an appropriate voltage to the LC cell.
A two-dimensional array of such cells, each cell individually addressable through a matrix of row and column electrodes, can be used to build up a display such as a video image, where each cell constitutes a pixel of information.
In one form of such an array, called an active matrix device, each pixel has an associated thin film transistor (TFT) switch, interconnecting the pixel with row and column electrodes. For example, the column electrode is connected to the source electrode of the TFT, the row electrode is connected to the gate electrode of the TFT, and the electrode defining the pixel is connected to the drain electrode of the TFT. In operation, the pixels are sequentially addressed, individually or row-by-row, through addressing signals supplied via the row electrodes, and during the addressing period the pixels are supplied with data signals via the column electrodes.
A disadvantage of such TFTs and certain other thin film switches is their sensitivity to light. Particularly under the large light flux to which the LCD's are subjected in a projection apparatus, this photosensitivity can result in significant current leakages through the switches.
In order to prevent this from happening, various arrangements have been proposed to shield the TFT's from incident light. For example, in U.S. Pat. No. 4,599,246, an opaque insulator film of, for example, germanium, is formed over the channel regions of the TFT's in an LCD array. In EPO 186 036, a light shielding layer of, for example, chromium is located behind an insulating layer adjacent the TFT's in an LCD array. In JA 63-41133, a thin film element for a display panel is sandwiched between light shielding layers of amorphous silicon-germanium or germanium. In JA 58-62622, a film of amorphous silicon covers the entire array of a liquid crystal display in order to shield the transistors from light, and also to reduce the chemical reaction between the liquid crystal and the metallic electrodes. This arrangement would only be suitable for an LCD operating in the reflective mode since the pixel electrodes are prevented from transmitting light by the placement of the protective layer.
In the other arrangements referred to, the light shielding layer is limited to the TFT's themselves, or even to the channel region of the TFT's, so that light cannot reach these areas directly. However, light may still reach these areas indirectly, for example, via scattering or reflection in the areas between the pixel electrodes (herein "interpixel areas").